The following is a tabulation of some prior art that presently appears relevant:
U.S. patentsU.S. Pat. No.Kind CodeIssue DatePatentee8,561,268B22013 Mar. 22Hortnagl2,867,8121959 Jan. 13Roth5,606,7791997 Mar. 4Lu
U.S. patent application PublicationsPublication NrKind CodePubl. DateApplicant2015/0033457A12015 Feb. 5Tryner2015/0113770A12015 Apr. 30Laatz